1. Field of the Invention
The invention relates generally to a process for producing binder-containing fibrous mats.
2. Discussion of the Related Art
Such fibrous mats are known in many different compositions and are moulded or shaped in the most varied ways into self-supporting moulded or shaped articles.
Thus, e.g. a process for the hot pressing of shaped articles of the aforementioned type is known, in which for the shaping process a stabilizing support is applied on at least one side between the shaping surface of the shaping tool or mould and the mat fleece, as disclosed in German publication DE-OS 37 21 663. This process is characterized in that the stabilizing support is constituted by a composite fibre layer absorbing tensile forces in the surface direction, which is substantially extension or elongation-free, but under the heat action within the hot press is plastically deformable and is hardened or solidified immediately after the shaping process to a substantially elongation-free, inflexible covering.
A fibrous fleece mixture of lignocellulose fibres and cotton fibres is used, which is mixed with heat-hardening binders. This fibrous fleece mixture is embedded between two stabilizing supports which are substantially elongation-free in the surface direction and prior to the shaping of this raw mat blank a union is formed between the individual laminations, so that during the shaping or pressing process, the tensile forces which occur can be absorbed in troublefree manner. The necessary adhesion between the flexible stabilizing supports and the surface of the fibrous fleece mixture results from the fact that a heat-resistant self-adhesion occurs in the temperature range 60.degree. to 150.degree. C. The stabilizing support applied to one or both sides of a mat fleece can be applied according to the prior art either by sprinkling on one or more layers of high-strength threads, or as a prefabricated form such as a tangled fibre fleece composite. The heat-resistant adhesion between the stabilizing support and the wood fibre mat is distributed over several fibre layers, in order to ensure the aforementioned tensile force transfer.
It is also no longer novel in this connection to process and construct the stabilizing support in such a way that its air permeability is retained both before and after the shaping of the fibrous mat to a shaped or moulded article. For this purpose the stabilizing support thickness is kept very small compared with that of the fibrous fleece. It is fundamentally assumed that such a stabilizing support is provided on either one or both surfaces of the wood fibre fleece, but not in the interior of the mat blank.
Another known fibrous mat type is characterized by the fact that for stabilizing in the shaping process and for the transfer of forces into the middle of the tangled fibre fleece; a firm fabric is inserted. The firm fabric permanently absorbs the tensile and shear forces in the manner of a tension network. The fabric embedded in the fibrous fleece mat is only reached by the tensile forces of the shaping tool if the forces have already been transferred from the shaping tool surfaces onto the fibrous fleece mat. This generally leads to considerable damage in the structure of the fibrous mat before the shaping tool has reached the stabilizing fabric insert allowing the insert to come into action.
Therefore research has been carried out with the aim of producing fibrous fleece mats comprising a substantially homogeneous mixture of cellulose or lignocellulose fibres, which are not loadable for the shaping process; and longer synthetic threads, or the like, which absorb the tensile and shear forces. Heretofore it has been found that, at least in mass production, the necessary homogeneous sprinkling onto a conveying member for producing the fibrous fleece, cannot be realized in a technical manner. The pouring characteristics of the different fibre types always lead to an undesired and substantially uncontrollable separation of the individual fibre types. The resulting quality reduction of the finished fibrous mat can be compensated only to a certain extent by an increased use of expensive synthetic fibres. The increased use of synthetic fibers apart from increasing the cost of the product, leads to the disadvantage of air impermeability of the fibrous mat.
The problem solved by the present invention is to provide a process for producing fibrous mats from a mixture of wood and textile fibres to produce a mat that has a statistically averaged, homogeneous distribution of synthetic fibres absorbing tensile and shear forces and a minimum synthetic fibre proportion within the mat and over the cross-section, or thickness, thereof, and in particular, a minimized synthetic fibre proportion in the surface region of the mat.